ResearchBlogging.orgWHILE MANY NICHES are ever-present in vertebrate-dominated ecosystems, sometimes we find an extinct group that was unique in some way. At various times in the past carnivores have evolved saber-teeth, but currently there are no saber-toothed predators (although the clouded leopard may be working on it). In a previous post I mentioned Simocyon, a cursorial generalized carnivore that retained arboriality in order to escape from larger predators, including the saber-toothed cats. Today I will write about Thylacoleo carnifex, the so-called marsupial lion.

The skull of T. carnifex is unusual (or, as I said to myself, “frickin’ bizarre”). The canines are diminished in size, but the incisors are enlarged into protuberant fang-like teeth. The premolars and molars are also reduced, except for the last pair of premolars, which is greatly enlarged into a shearing blade. These odd teeth are obvious in a photo of a juvenile’s mandible and a slightly less well-preserved adult skull. Another unusual trait is a retained semi-opposable thumb with a large claw, seen in this skeleton.

T. carnifex‘s tooth morphology is so unusual that its diet was debated for some time, with some thinking it was adapted to chopping up plants. With more information about the full skeleton the paleontological community generally agreed that T. carnifex was adapted for hypercarnivory, rather like modern cats. Cats have decreased premolars and enlarged carnassials for shearing meat. While T. carnifex modified different teeth, the end product was similar. Parallels have been drawn between T. carnifex and saber-toothed cats such as Smilodon. T. carnifex was not saber-toothed (the saber-toothed marsupial was Thylacosmilus) but shares a similar body build with Smilodon. An analogy has also been drawn between Smilodon and the bears, which have a robust build. Wroe, Lowry, and Anton set out to analyze a series of morphological traits across 19 placental and 10 marsupial species and determine whether the morphology of these predators was convergent and what analogies can be drawn.

By mapping the proportions of various bones, they were able to construct a phenogram, which clusters organisms together according to morphology. Most groups cluster in a manner consistent with their relationships, so canines cluster together, most felines together (including the scimitar-toothed saber-toothed cats), and marsupials in their own group. However, the two smilodons examined (Smilodon fatalis and Smilodon populator) cluster with T. carnifex as a morphological “sister group” to the bears, with T. carnifex being the most bear-like of the group. The traits responsible for this are relatively shortened bones in the distal forelimbs, a large distance between the bicipital tuberocity and proximal end of the radius (which translates to mechanical advantage for the biceps brachii), and high robusticity with a short, heavy torso. These changes would decrease these animals’ cursorial ability, but increase stability and forelimb strength. Bears probably need these traits because they utilize a wide variety of foraging behaviors, from climbing to digging to overturning obstacles.

While smilodons and T. carnifex share some bear-like traits, they differ from bears in their dental adaptations, which categorize these two groups as hypercarnivores. Bears, on the other hand, have a more generalized dentition that in most species has even become adapted for omnivory and sometimes folivory.

The bear-like traits shared by smilodons and T. carnifex probably are related to ambush predation upon large, relatively non-nimble prey. The strong forelimbs and prominent dewclaws would enable these predators to grapple prey. The authors note that bears often capture prey by a short chase followed by rearing on their hind legs and seizing the animal’s front end, pinning it. This is consistent with previous conclusions regarding Smilodon‘s means of predation (similar to that of dirk-toothed nimravids, discussed here). The killing bite itself would be different, since Smilodon has dirk-teeth while T. carnifex has enlarged incisors more similar in size to typical predator’s canines. Previous work shows that T. carnifex has a bite strength disproportional to its size, which may have enabled it to achieve rapid kills.

This research clarifies past claims of bear-like traits among the dirk-toothed saber-toothed cats, shows that T. carnifex converged upon this morphology while not evolving saber-teeth, and demonstrates the existence of a unique extinct predator morphology. Thylacosmilus, the extinct saber-toothed marsupial, also had a stocky build and may have shared this morphology. Perhaps future work will examine the similarities and differences between Thylacosmilus, Thylacoleo, and the smilodons.

Brian Switek covered Thylacoleo last year on the previous incarnation of his blog, Laelaps (now at its location at ScienceBlogs). Darren Naish at Tetrapod Zoology also coincidentally wrote about Thylacoleo today, but mentioning different research! He refers to the studies on Thylacoleo‘s bite force.

WROE, S., LOWRY, M., ANTON, M. (2008). How to build a mammalian super-predator. Zoology, 111(3), 196-203. DOI: 10.1016/j.zool.2007.07.008